The present invention is an improvement to metal strip, floatation or floater furnaces. Strip floatation furnaces are well known in the art. In such furnaces, metal strip (ferrous or non-ferrous, thick or thin gauge) is heat treated (generally annealing although other heat treat processes such as spherodizing, aging or solution treating are performed) in a furnace through which the strip is pulled at a relatively fast speed (as high as 600 feet per minute). The strip is either uncoiled from coils where it is wound and passes through the furnace or the strip can enter the furnace directly from the mill after passing through looping towers (i.e., an accumulator). After exiting the furnace, the strip typically passes through additional looping towers before being immersed in one or more baths or tanks after which it is wound into coils. Alternatively, the strip may be immediately wound into coils after leaving the furnace.
The floater furnace is a long, box construction which essentially contains pressure pads extending in rows spaced along the length of the furnace extending transversely to the direction of strip movement and situated above and below the strip. The pads direct jet streams of atmosphere in a particular pattern against the strip to suspend or float the strip so that no mechanical contact with the strip can occur. This avoids any strip marking or reduction in gauge thickness which could otherwise occur if strip rolls and the like were used to support the strip within the furnace. Sometimes (and typically) nozzles (formed in pipes extending as rows) are added between the pads to provide sufficient flow of furnace atmosphere or wind so that the strip is raised to the appropriate temperature whereat the heat treating operation can be performed. Depending on the heat treat process, the furnace is typically divided into zones and the temperature (and to a lesser extent the atmosphere) is controlled in each zone. For example, the strip is preheated, heated to the heat treating temperature, and cooled in zones as it travels through the furnace from the entrance to the exit end.
The manner in which the atmosphere is distributed to the pressure pads and nozzle pipes and after impingement, the manner in which the "spent" atmosphere is collected and refined prior to being introduced again as "fresh" atmosphere to the pressure pads and nozzle pipes will vary somewhat from one manufacturer to the next. However, the most pertinent prior art is believe to be the assignee Surface Combustion Inc.'s own floater design which has long been recognized and accepted within the industry.
In the Surface floater furnace, the pressure pad and nozzle pipes are connected at their transverse ends to a manifold or header which in turn is ducted to a fan housing containing a fan. Rotation of the fan impeller causes wind or furnace atmosphere to be pumped under pressure through the headers to the pipes and pads for supporting and effecting heat transfer with the strip as described. A return duct above the pads and pipes and near each end of each furnace "zone" collects the "spent" atmosphere or wind which is pulled by the fan into the fan housing for subsequent distribution to the headers, etc. Within the return duct adjacent its open end are heat transfer tubes which "rejuvenate" the spent atmosphere as it is pulled by the fan through the return duct. This arrangement provides an efficient, compact return loop which has commercially proven itself and which is utilized in the present invention.
The furnace enclosure surrounding and containing the atmosphere distribution system described above is a conventional, furnace construction in which fibrous furnace insulation is applied to the interior of a metal casing. In the Surface design, this insulation is covered with a metal liner to prevent the atmosphere from contacting the insulation, picking up refractory fibers and other foreign matter and transferring them to the strip. That is, the spent jets will circulate randomly throughout the furnace enclosure before being drawn into the return duct. During circulation the wind will impinge the furnace enclosure walls and (if not lined) pick up or entrain refractory particles (and other foreign matter) which will then be pumped back by the fan through the pipes and pads onto the metal strip. To prevent this from occurring, Surface installs a metal, alloy liner over the insulation. Furthermore, the inner metal or panel liner, because of differential temperatures from the furnace enclosure walls, has to be equipped with expansion joints and strips and the like to allow for expansion.
Lining the entire furnace enclosure is relatively expensive (not only from a material cost standpoint but also from an engineering expense since each furnace must be "custom" lined to provide for thermal expansion and contraction). In time the maintenance of the furnace eventually becomes somewhat significant since the expansion strips can eventually fail and expose the insulation to the atmosphere. Also, the blanket fibers can break down and settle. In either instance, the wind within the furnace can entrain foreign matter and impinge the strip. Further, the headers are positioned closely adjacent the furnace enclosure side walls and it becomes difficult to gain access to the furnace enclosure, both for visual inspection to determine failure in the first instance and for repair once the failure is detected.
Other approaches either adopted or suggested by others to address the concerns discussed above have been to utilize other types of furnace insulation coated with a special refractory mortar. However, the mortar is not nearly as durable as the metal liner described above and mortar/refractory arrangements have their own characteristic problems. Other approaches have been to use special hardened ceramic fibre coatings which are more or less "sprayed" onto the insulation producing a smooth surface similar to that of castable refractories. Such insulation is expensive and may crack in time. The repairs then become significant since entire sections of refractory may have to be removed and replaced. Until the present invention, it is believed the metal liner discussed above represented the most durable and functionally the best solution to the problem.